The following books and web sites have good treatments of object persistence:
An excellent treatment of implementing OO models in relational databases.
A discussion of the CORBA Persistence architecture. Interesting as much for the discussion of politics behind such committees, as the technical material.
CORBA Persistence Service Specification, Object Management Group (OMG), at http://www.omg.org.
PJava: Orthogonal Persistence for Java, at http://www.dcs.gla.ac.uk/pjava/. Look especially for the design paper entitled "Design Issues for Persistent Java: a Type-Safe, Object-Oriented, Orthogonally Persistent System."
Object Database Management Group (ODMG): http://www.odmg.org.
This paper shows you that there's much more to object identity than meets the eye. Download from ftp://ftp.netcom.com/pub/hb/hbaker/ObjectIdentity.html.